linux-kernel - RE: [PATCH] Drivers: hv: Change flag to write log level in panic msg to false

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Message-ID: <MW2PR2101MB10520D5E1FDE45C5DF03D073D7930@MW2PR2101MB1052.namprd21.prod.outlook.com>
Date:   Fri, 26 Jun 2020 20:53:25 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Joseph Salisbury <Joseph.Salisbury@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] Drivers: hv: Change flag to write log level in panic msg
 to false

From: Joseph Salisbury <joseph.salisbury@...rosoft.com> Sent: Friday, June 26, 2020 10:48 AM
> 
> When the kernel panics, one page worth of kmsg data is written to an allocated
> page.  The Hypervisor is notified of the page address trough the MSR.  This
> panic information is collected on the host.  Since we are only collecting one
> page of data, the full panic message may not be collected.
> 
> Each line of the panic message is prefixed with the log level of that
> particular message in the form <N>, where N is the log level.   The typical
> 4 Kbytes contains anywhere from 50 to 100 lines with that log level prefix.
> 
> hv_dmsg_dump() makes a call to kmsg_dump_get_buffer().  The second argument in
> the call is a bool described as: ‘@syslog: include the “<4>” Prefixes’.
> 
> With this change, we will not write the log level to the allocated page.  This
> will provide additional room in the allocated page for more informative panic
> information.

Let me suggest tightening the commit message a bit, with focus on the "what"
and "why" rather than the details of the code change.  Also use imperative
voice per the Linux kernel guidelines:

When the kernel panics, one page of kmsg data may be collected and sent to
Hyper-V to aid in diagnosing the failure.  The collected kmsg data typically
contains 50 to 100 lines, each of which has a log level prefix that isn't very
useful from a diagnostic standpoint.  So tell kmsg_dump_get_buffer() to not
include the log level, enabling more information that *is* useful to fit in the page.

> 
> Requesting in stable kernels, since many kernels running in production are
> stable releases.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Joseph Salisbury <joseph.salisbury@...rosoft.com>
> ---
>  drivers/hv/vmbus_drv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 9147ee9d5f7d..d69f4efa3719 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1368,7 +1368,7 @@ static void hv_kmsg_dump(struct kmsg_dumper *dumper,
>  	 * Write dump contents to the page. No need to synchronize; panic should
>  	 * be single-threaded.
>  	 */
> -	kmsg_dump_get_buffer(dumper, true, hv_panic_page, HV_HYP_PAGE_SIZE,
> +	kmsg_dump_get_buffer(dumper, false, hv_panic_page, HV_HYP_PAGE_SIZE,
>  			     &bytes_written);
>  	if (bytes_written)
>  		hyperv_report_panic_msg(panic_pa, bytes_written);
> --
> 2.17.1

With the commit message changes,

Reviewed-by: Michael Kelley <mikelley@...rosoft.com>